1. Field of the Invention
The present invention relates to a heart-sound analyzing apparatus which can determine a timing when aortic valve closes, and particularly to such an apparatus which can iteratively determine a timing when aortic valve closes.
2. Related Art Statement
A second heart sound II includes a component resulting from closure of aortic valve. For a certain diagnosis, it is useful to determine a timing when aortic valve closes. For example, a timing of closure of aortic valve may be utilized to measure a pulse-wave propagation velocity PWV at which a pulse wave propagates in a living subject. Pulse-wave propagation velocity PWV can be determined by dividing a propagation time DT needed for a pulse wave to propagate between two reference positions, by a propagation distance between the two reference positions. Therefore, as propagation time DT increases, an accuracy of determination of propagation velocity PWV increases. Since the closure of aortic valve occurs at the upstream-side end of blood flow, the distance of propagation of pulse wave can be increased by determining the timing of closure of aortic valve, and accordingly a highly accurate propagation velocity PWV can be determined based on the increased propagation distance. In addition, since the ejection of blood from the heart ends with the closure of aortic valve, the timing of closure of aortic valve needs to be determined to calculate an ejection time.
The second heart sound II includes not only the component IIA resulting from the closure of aortic valve but also a component IIP resulting from closure of pulmonary valve. Though, generally, the component IIP follows the component IIA, a time interval between the component IIA and the component IIP is changed by respiration of a subject. More specifically described, the above-indicated time interval is increased and the two components IIA, IIP are separated from each other, when the subject is inspiring, but the time interval is decreased and the first half portion of the component IIP overlaps the second half portion of the component IIA, when the subject is expiring. Though the component IIA needs to be analyzed to determine the timing of closure of aortic valve, it is difficult to analyze the component IIA overlapped by the component IIP. To solve this problem, it has conventionally been practiced to ask the subject to control his or her respiration.
However, it is difficult for a subject to control continuously his or her respiration for the purpose of determining iteratively a timing of closure of aortic valve. In particular, it is difficult for a patient under a surgical operation to control continuously his or her respiration.
It is therefore an object of the present invention to provide a heart-sound analyzing apparatus which can accurately and iteratively determine a timing when aortic valve closes.
The above object has been achieved by the present invention. According to a first feature of the present invention, there is provided a heart-sound analyzing apparatus, comprising a heart-sound microphone which is adapted to be worn on a living subject to iteratively detect a second heart sound II of the subject; a respiration-synchronous-signal detecting device which iteratively detects a respiration-synchronous signal of the subject; an inspiration-expiration judging means for judging, based on each of the respiration-synchronous signals iteratively detected by the respiration-synchronous-signal detecting device, whether the subject is in an inspiring state or in an expiring state; and an aortic-valve-closing-timing determining means for iteratively determining a timing when an aortic valve of a heart of the subject closes, based on each of a plurality of second heart sounds II which are iteratively detected by the heart-sound microphone in a plurality of time intervals, respectively, in each of which the subject is judged as being in the inspiring state by the inspiration-expiration judging means.
In the present heart-sound analyzing apparatus, the inspiration-expiration judging means judges, based on each of the respiration-synchronous signals detected by the respiration-synchronous-signal detecting device, whether the living subject is in an inspiring state or in an expiring state, and the aortic-valve-closing-timing determining means determines a timing when the aortic valve closes, based on a second heart sound II which is detected by the heart-sound microphone when the subject is in the inspiring state. Therefore, the present apparatus can accurately and iteratively determine a timing when the aortic valve closes, without needing to force the subject to control his or her respiration.
According to a second feature of the present invention, the aortic-valve-closing-timing determining means determines a timing when the aortic valve closes, based on the last one of a plurality of second heart sounds II which are detected by the heart-sound microphone in each of the plurality of time intervals in each of which the subject is judged as being in the inspiring state by the inspiration-expiration judging means. In the present heart-sound analyzing apparatus, the aortic-valve-closing-timing determining means determines a timing when the aortic valve closes, based on the last second heart sound II wherein the component IIA and the component IIP are most largely separated from each other. Therefore, the present apparatus can more accurately determine a timing when the aortic valve closes.
According to a third feature of the present invention, the heart-sound analyzing apparatus further comprises a smoothing means for smoothing, by differentiation, a waveform of the second heart sounds iteratively detected by the heart-sound microphone; and a squaring means for squaring respective amplitudes of a plurality of data points on the smoothed waveform that are determined with respect to a base line of the smoothed waveform, and the aortic-valve-closing-timing determining means iteratively determines a timing when the aortic valve closes, based on the amplitudes squared by the squaring means. Each of the second heart sounds II detected by the heart-sound microphone includes a low-frequency noise, and has an alternating waveform including positive amplitudes and negative amplitudes on both sides of a base line thereof. In the present heart-sound analyzing apparatus, the smoothing means smoothes, by differentiation, a waveform of the second heart sounds detected by the heart-sound microphone, and thereby provides a smoothed waveform in the form of a differentiated waveform which shows a clear change of amplitudes. In addition, the squaring means squares respective amplitudes of respective data points on the smoothed waveform that are determined with respect to a base line of the smoothed waveform, and thereby provides a squared waveform having the thus squared amplitudes on only the positive side of the base line. And, the aortic-valve-closing-timing determining means determines a timing when the aortic valve closes, based on the squared waveform provided by the squaring means. Therefore, the present apparatus can determine a more accurate timing when the aortic valve closes.
According to a fourth feature of the present invention, the heart-sound analyzing apparatus further comprises an electrocardiograph which includes a plurality of electrodes adapted to be worn at a plurality of locations on the subject and which detects, through the electrodes, an electrocardiogram of the subject, and the aortic-valve-closing-timing determining means comprises means for iteratively determining, based on a prescribed point of each of respective first half portions of a plurality of T-waves of the electrocardiogram detected by the electrocardiograph, a start point of a corresponding one of a plurality of waveform-process intervals in each of which the aortic-valve-closing-timing determining means is to seek a timing when the aortic valve closes. A second heart sound II occurs during a certain time duration around the second half portion of each T-wave of the electrocardiogram. Therefore, the present heart-sound analyzing apparatus identifies, as the component IIA, the first change of the waveform of the second heart sound II after the start point of each waveform-process interval, and does not erroneously identify a different change as the component IIA. Thus, the present apparatus can more accurately determine a timing when the aortic valve closes.
According to a fifth feature of the present invention, there is provided a system for obtaining information relating to a propagation velocity at which a pulse wave propagates along an artery of a living subject, the system comprising a heart-sound analyzing apparatus according to any one of the above-described first to fourth features; a pulse-wave detecting device which is adapted to be worn on the subject to detect the pulse wave which propagates along the artery of the subject; and a pulse-wave-propagation-velocity-relating-information obtaining means for obtaining the information based on the timing when the aortic valve closes, determined by the aortic-valve-closing-timing determining means of the heart-sound analyzing apparatus, and a timing when a prescribed point of the pulse wave is detected by the pulse-wave detecting device.
The information relating to the pulse-wave propagation velocity may be the pulse-wave propagation velocity itself, or a pulse-wave propagation time.
In the present pulse-wave-propagation-velocity-relating-information obtaining system, the aortic-valve-closing-timing determining means of the heart-sound analyzing apparatus accurately determines a timing when the aortic valve closes, and the pulse-wave-propagation-velocity-relating-information obtaining means obtains the pulse-wave-propagation-velocity-relating information, based on the accurately determined timing when the aortic valve closes, and a timing when the prescribed portion of the pulse wave is detected by the pulse-wave detecting device. Therefore, the present system can obtain accurate pulse-wave-propagation-velocity-relating information.